Simulation of the phase change and deposition of inhaled semi-volatile liquid droplets in the nasal passages of rats and humans

Inhaled airborne droplets may undergo phase change as they travel through the respiratory tract. Droplet evaporation and condensation depends on the physicochemical and thermodynamic properties of the aerosol and will impact deposition in the respiratory airways. The deposition behavior of four fragrance aerosols (d-limonene, linalool, benzaldehyde, and benzyl acetate) with different vapor pressures was simulated in anatomically accurate computational fluid dynamics (CFD) models of the rat and human nasal passages. Steady-state inspiratory airflow was simulated in each species at resting breathing rates. Droplet transport and deposition was simulated using Lagrangian particle tracking. Droplet size evolution due to phase change was governed by molecular diffusion to or from the droplet surface and was driven by the difference in partial pressures between the droplet surface and the surrounding environment. CFD predictions revealed significant evaporation of these fragrance materials in the nasal passages with evaporation rates dependent on the saturation vapor pressures of the substances. In the human nose, complete evaporation was observed for d-limonene droplets with initial diameters ≤6 µm, benzaldehyde droplets ≤4 µm, and benzyl acetate and linalool droplets <2 µm. The minimum droplet size for complete evaporation was smaller in the rat nose due to higher deposition in the anterior nose. Larger droplets did not completely evaporate but decreased in size, thereby reducing deposition by inertial impaction in the nasal passages of each species. Quantification of droplet deposition in the nose has significant implications for lung dosimetry since vapor concentrations and droplet characteristics are altered during transport through the upper respiratory tract.